In conventional digital cellular systems such as GSM (Global System for Mobile communications) and PDC (Personal Digital Cellular), channel coding is used in the air interface to reduce the bit error rate (BER). Channel coding is also commonly referred to as error control coding, examples of which include block coding and convolutional coding. These and other error control coding techniques are well known in the art.
In some conventional systems, constant channel coding rates are respectively assigned to speech and data communications. The channel coding rate of a given channel coding scheme refers to the amount of redundant bits added to the actual message bits by the coding scheme in order to implement the desired level of error control. Higher channel coding rates can provide a lower BER for a certain quality at the cost of lower throughput, and lower channel coding rates permit higher throughput at the possible cost of higher BER for a certain quality.
One disadvantage with assigning a constant channel coding rate to, for example, all data communications, is that channel capacity might be wasted on a given link between a mobile station (also referred to as a mobile unit) and a base transceiver station if that link has good quality. In such a situation, the link may not require as high a channel coding rate as is provided by the constant channel coding rate, so that at least some of the channel coding bits are unnecessary overhead because a good quality link needs little or no channel coding to achieve an acceptable BER. For poor connections between mobile units and base transceiver stations, the opposite can occur. That is, in order to achieve an acceptable BER, a poor connection may need a higher channel coding rate than the assigned channel coding rate.
One conventional technique directed to solving this problem is to provide multiple modulation and channel coding schemes, and utilize a link adaptation algorithm which attempts to maximize the throughput on the individual radio links between the respective mobile stations and the base transceiver station. This is done by adaptively choosing, from the multiple modulation and coding schemes, the one scheme that achieves the highest throughput on a given link based on the time varying quality of that link. The throughput for each mobile unit is thereby adapted to the "radio" situation of its link, namely the propagation and interference conditions. If the link quality is good (good propagation and interference conditions), then the link adaption algorithm will assign a channel coding scheme having a lower coding rate, while a channel coding scheme having a higher coding rate will be assigned to the link if poor propagation and interference conditions are present.
Conventional link adaptation approaches such as described above typically receive as inputs a number of quality characteristics for each link, for example, (1) downlink and uplink measures of received signal power, (2) downlink and uplink measures of received interference, and (3) downlink and uplink measures of BER.
The aforementioned use of multiple modulation and channel coding schemes with a link adaptation algorithm permits a given digital cellular system to adapt to its operating environment. Because it is difficult to find an "optimal" modulation and coding scheme that will fit every operating environment, it is likely that a modulation and coding scheme optimized for a first environment with relatively favorable propagation and interference conditions will provide insufficient error control in a second environment with relatively poor propagation and interference conditions. Conversely, a modulation and channel coding scheme that is optimized for the second, relatively poorer, environment will likely provide error control that is unnecessary overhead in the first environment. The above-described use of multiple modulation and channel coding schemes which are applied on a per link basis by a link adaptation algorithm is better suited for use with different environments than is the technique of assigning a constant modulation and channel coding scheme without regard to the operating environment.
However, the link adaptation approach typically requires that the aforementioned measures of link quality be measured and reported at regular intervals typically ranging from one to fifty times per second. These reports, which are needed as inputs to the link adaptation algorithm, must of course be transmitted over the air interface, thus requiring a large and expensive amount of overhead in the air interface for transmission of the downlink measurements from the mobile units to the base transceiver stations. Moreover, a more complex communication protocol is required between the mobile units and the base transceiver stations in order to handle the frequent measurements, the corresponding measurement reports, and the potentially frequent changes of the modulation and channel coding scheme in response to the frequently reported measurements. Frequent changes in the modulation and channel coding schemes requires additional complexity in the protocol between the mobile units and the base stations, as well corresponding complexities in the computational capabilities of the mobile units and base stations.
Another disadvantage of the aforementioned link adaptation approach is that it tends to prevent full utilization of conventional power control techniques which are designed to reduce interference and increase the battery life of the mobile units. Whereas the link adaptation approach tries to maximize the throughput of each individual link regardless of the throughput of any other neighboring links, conventional power control techniques attempt to achieve more or less the same quality and thereby the same throughput for all links in the system. Such power control techniques attempt to improve the quality of relatively poor quality links by degrading the quality of relatively high quality links. For example, transmission power in a good quality link will typically be reduced, while transmission power in a poor quality link will typically be increased. In contrast, the link adaptation approach will typically fix the power level of a given link to the maximum allowed power level.
It is therefore desirable to provide, in a digital cellular system, capability of adjusting throughput by adaptively selecting from multiple modulation and channel coding schemes, without the disadvantageous overhead of the conventional link adaptation approaches, and without disadvantageously interfering with conventional power control techniques. This is achieved according to the present invention by providing a relatively slowly adaptive technique which selects a modulation and channel coding scheme from a plurality of possible choices, and applies the selected modulation and channel coding scheme on a cell-level basis.